1. Field of the Invention
The present invention relates to apparatuses and methods for driving display devices, especially to improvement in the inversion driving technique.
2. Description of the Related Art
Liquid crystal displays often suffer from the “burn-in” effect, which is known as a phenomenon in which applying a DC voltage to pixels within a liquid crystal display causes serious degradation of the lifetime of liquid crystal material filled in the pixels.
In order to avoid the “burn-in” effect, liquid crystal displays often adopt an inversion driving technique (or an alternating driving technique). The inversion driving technique involves periodically inverting the polarity of the data signal applied to each pixel. The inversion driving technique effectively reduces the DC component of the voltage across the liquid crystal capacitance within the pixel, and thereby avoids the “burn in” effect.
The inversion driving technique is schematically classified into common constant driving and common inverting driving. The common constant driving designates a driving method which inverts the polarities of data signals applied to the pixels, with the potential of the common electrode (or the back electrode) kept constant; the potential of the common electrode is referred to as the common potential VCOM, hereinafter. The common inversion driving, on the other hand, designates a driving method which inverts both of the polarities of data signals and the potential of the common electrode.
The common constant driving is advantageous in terms of the stability of the common potential VCOM over the common inversion driving. As known in the art, the stability of the common potential VCOM is important for reducing flicker. Therefore, the present invention is directed to the common constant driving.
One issue of conventional common constant driving techniques is that drive circuits developing data signals are required to operate on a high power source voltage. A typical liquid crystal driver adopting the common constant driving requires feeding drive circuits with a power supply voltage equal to or higher than twice of maximum voltages applied to pixels. For the case that the liquid crystal capacitances are supplied with a voltage of 5 V at a maximum, the drive circuits require a power supply voltage of 10 V.
Operating drive circuits on a high power supply voltage is accompanied by two disadvantages: Firstly, circuit elements within the drive circuits are required to have a high withstand voltage, specifically, equal to or higher than twice of the maximum voltages applied to the pixels. Another disadvantage is the increase in the power consumption. The power consumption of the drive circuits proportionally increases as the power supply voltage, and therefore, the increase in the power supply voltage undesirably increases the power consumption.
Japanese Laid-Open Patent Application (JP-A-Heisei, 10-62744) discloses LCD driver architecture for overcoming these disadvantages. FIG. 1 is a block diagram illustrating the conventional LCD driver architecture. The conventional LCD driver deals with the above-described problem through separating the circuitry for developing data signals of the positive polarity with respect to the common potential VCOM from the circuitry for developing data signals of the negative polarity with respect to the common potential VCOM and from each other.
More specifically, the LCD drive shown in FIG. 1 is composed of an input-side polarity switch circuitry 101, a grayscale voltage generator circuit 102, a set of positive-side driver circuitries 103, a set of negative-side driver circuitries 104, an output-side polarity switch circuitry 105, a polarity switch control circuit 106, and a timing controller circuit 107.
The input-side polarity switch circuitry 101 forwards pixel data associated with respective pixels within the LCD panel to desired ones of the positive-side driver circuitries 103 and the negative-side driver circuitries 104 in response to the polarities of data signals supplied to the respective pixels.
The grayscale voltage generator circuit 102 is composed of a positive grayscale voltage generator 102a, and a negative grayscale voltage generator 102b. The positive grayscale voltage generator 102a develops a set of grayscale voltages of the positive polarity with respect to the common potential VCOM, and the negative grayscale voltage generator 102b develops a set of grayscale voltages of the negative polarity with respect to the common potential VCOM.
The positive-side driver circuitries 103 develop data signals of the positive polarity with respect with the common potential VCOM 103, using the grayscale voltages received from the positive grayscale voltage generator 102a. When the common potential VCOM is 5 V and the maximum voltages applied to the pixels is 5V, for example, the positive-side driver circuitries 103 develop data signals having signal levels of 5 to 10 V. The positive-side driver circuitries 103 are each composed of a latch circuit 103a, a level shifter 103b, a D/A converter 103c, and a positive drive circuit 103d. In order to develop data signals having signal levels of 5 to 10 V, the positive drive circuits 103d are fed with a power supply voltage of 10 V. The positive drive circuits 103d are each typically composed of an operation amplifier.
Correspondingly, the negative-side driver circuitries 104 develop data signals of the negative polarity with respect with the common potential VCOM 103, using the grayscale voltages received from the negative grayscale voltage generator 102b. When the common potential VCOM is 5 V and the maximum voltages applied to the pixels is 5V, for example, the negative-side driver circuitries 103 develop data signals having signal levels of 0 to 5 V. The negative-side driver circuitries 104 are each composed of a latch circuit 104a, a level shifter 104b, a D/A converter 104c, and a negative drive circuit 104d. In order to develop data signals having signal levels of 0 to 5 V, the negative drive circuits 104d are fed with a power supply voltage of 5 V. The negative drive circuits 104d are each typically composed of an operation amplifier.
The output-side polarity switch circuitry 105 forwards the data signals developed by the positive-side driver circuitries 103 and the negative-side driver circuitries 104 to desired ones of the output terminals 108. The output terminals 108 are connected with data lines within an LCD panel, and the data signals are fed to the data lines through the output terminals 108.
The output-side polarity switch 105 is provided with switches 105a for precharging the output terminals 108 to half of the LCD drive voltage VLCD, that is, the potential of 5 V.
The feature of the LCD driver shown in FIG. 1 is that the LCD driver is composed of the positive-side driver circuitries 103, dedicated for developing the positive data signals, and the negative-side driver circuitries 104, dedicated for developing the negative data signals. This architecture only requires providing the negative-side driver circuitries 104 with a power supply voltage comparable to the maximum voltage across the pixels; the negative-side driver circuitries 104 do not require to be fed with a power supply voltage of twice or more of the maximum voltage across the pixels. This effectively reduces the power consumption of the LCD driver.
Another advantage is that the circuit elements within the positive drive circuits 103d and the negative drive circuits 104d are applied with voltages comparable to the maximum voltage across the pixels at a maximum. This is because the output terminals 108 are precharged to the half level of the liquid crystal drive voltage VLCD by the switches 105a. The LCD drive architecture shown in FIG. 1 eliminates the need for designing the positive drive circuits 103d and the negative driver circuits 104d to have a high withstand voltage.
From the inventors' study, however, there is room for further reducing the power consumption for the LCD driver shown in FIG. 1. Although the above-described LCD driver lowers the power supply voltage supplied to the negative drive circuits 104, the negative drive circuits 104 still requires to operate a high power supply voltage.
FIG. 2 is a diagram illustrating the drawback of the conventional LCD driver shown in FIG. 1. FIG. 2 shows an example assuming that the general power source of the LCD driver develops a power supply voltage of 3 V, the common potential VCOM is 5 V, and the maximum voltage across the pixels is also 5 V. In this case, the negative drive circuits 104d are designed to output data signals having signal levels of 0 to 5 V. This requires feeding a power supply voltage of 5 V to the negative drive circuits 104d. This requirement is easily satisfied by doubling the power supply voltage developed by the general power source, and stepping down the doubled power supply voltage to develop a power supply voltage of 5 V.
The positive driver circuits 103d, on the other hand, are designed to output data signals having signal levels of 5 to 10 V. This requires quadrupling the power supply voltage developed by the general power source, and stepping down the quadrupled power supply voltage to develop a power supply voltage of 10 V.
Although reducing the power supply voltage fed to the negative drive circuits 104d down to 5 V, the architecture shown in FIG. 5 requires feeding the power supply voltage as high as 10 V. This is undesirable for reducing the power consumption.